DESCRIPTION

The Elliptic Curve library provides a number of different
implementations through a single common interface. When constructing a
curve using EC_GROUP_new (see EC_GROUP_new(3)) an implementation method
must be provided. The functions described here all return a const
pointer to an EC_METHOD structure that can be passed to EC_GROUP_NEW.
It is important that the correct implementation type for the form of
curve selected is used.
For F2^m curves there is only one implementation choice, i.e.
EC_GF2_simple_method.
For Fp curves the lowest common denominator implementation is the
EC_GFp_simple_method implementation. All other implementations are
based on this one. EC_GFp_mont_method builds on EC_GFp_simple_method
but adds the use of montgomery multiplication (see
BN_mod_mul_montgomery(3)). EC_GFp_nist_method offers an implementation
optimised for use with NIST recommended curves (NIST curves are
available through EC_GROUP_new_by_curve_name as described in
EC_GROUP_new(3)).
The functions EC_GFp_nistp224_method, EC_GFp_nistp256_method and
EC_GFp_nistp521_method offer 64 bit optimised implementations for the
NIST P224, P256 and P521 curves respectively. Note, however, that these
implementations are not available on all platforms.
EC_METHOD_get_field_type identifies what type of field the EC_METHOD
structure supports, which will be either F2^m or Fp. If the field type
is Fp then the value NID_X9_62_prime_field is returned. If the field
type is F2^m then the value NID_X9_62_characteristic_two_field is
returned. These values are defined in the obj_mac.h header file.

RETURN VALUES

All EC_GFp* functions and EC_GF2m_simple_method always return a const
pointer to an EC_METHOD structure.
EC_METHOD_get_field_type returns an integer that identifies the type of
field the EC_METHOD structure supports.